Electricity meters are commonly used by electrical utilities to measure, for example, electrical power flowing between a commercial power source and customer loads. Within each meter of present electromechanical design, there is a disk which is magnetically coupled to the power line voltages and currents being monitored. The disk rotates at a speed proportional to power flowing through the line and meter. To initially conform the meter to utility standards, and to correct the meter as a result of drift that tends to occur over time, the meter must be initially, and thereafter from time to time, calibrated.
An electromechanical polyphase meter has up to eight adjustments for calibration, including mechanical adjustments of screws which alter the positions of magnets and other mechanisms that, together with an electromagnetic field produced by the measured electricity, drive the disk at a particular speed of rotation for a given power load. The adjustments are made initially at the factory, then, if necessary, by the customer (i.e., the utility company) to meet utility standards and, thereafter, from time to time, to compensate the meter for drift. Adjustment usually is carried out manually, although automatic calibration systems with servo-driven screw adjusters have been implemented in factories.
Recently developed solid state meters, some of which are microprocessor controlled, currently becoming adopted by the electrical utilities, have the capability of carrying out various measurement functions within a single meter at higher levels of accuracy than previously realized using electromechanical meters. Solid state electricity meters are based upon several different signal processing techniques for determining energy usage parameters, such as line power, as a function of measured voltage and current, and there are a number of different manufacturer design approaches, although the industry is moving in the direction of standardization. For a survey of solid state electricity metering systems, attention is directed to an article entitled "Solid-State Metering", by Gorzelnik, in Electrical World, March, 1988, pp. 47-52.
In any type of solid state electricity meter, three parameters that may be available to the utility for adjustment are gain, offset, and phase. For example, in a meter of a type having line current and voltage sensors for obtaining line current and voltage measurements on each phase and a multiplier for multiplying the pairs of voltage and current measurements together taking into account the phase angle between them to obtain energy usage parameters, gain adjustments are needed on either the voltage or current signals of each voltage/current pair to equalize (balance) meter measurement on all system power phases. A single "overall" gain adjustment is further needed to compensate for electronic gain errors which affect all meter phase (element) inputs equally (caused particularly by initial errors in the voltage or current reference needed by any electronic power measuring device). The overall gain and element balance adjustments are commonly adjusted at a line current of 12.5 or 15 percent of full scale current, a level referred to herein as "Test Amperage". Separate resistance-capacitance network adjustments calibrate the meter phase angle response for errors in the current and voltage sensors as well as in the multiplier. A separate offset adjustment circuit supplies a small constant current or voltage to the multiplier to cancel the effect of offset errors. The offset is commonly adjusted to correct power measurement at a low current equal to ten percent of Test Amperage. Various meters have additional or fewer calibration adjustments available. The adjustment methodology used for the factory adjustment may be different from that provided to the customer utility. Some adjustment methods permit adjustment only at the factory.
Several techniques for carrying out the calibration adjustments just described include potentiometers, eleotromechanically switched resistor networks, and selected discrete resistors. Potentiometers are convenient to use but are difficult to automate. Even more significantly, however, the sliding contact between resistive elements and the wiper of the potentiometer tends to be unreliable. Mechanical switches, such as rotary or DIP-type switches, for switching the individual resistors of a network, are more reliable, but they are inconvenient. Also, since a large number of mechanical switch contacts must be provided to carry out calibration, the cost is high. Calibration using selected discrete resistors, although very reliable, is inconvenient, since unsoldering at least one resistor is required. Then, it is necessary to determine the value of, locate, and then install, at least one new resistor. Similarly, jumpers for interconnecting resistors are not easily installed or repositioned, since soldering and unsoldering, or some other irreversible mechanical operation, is required.
Each of the mentioned calibration techniques unfortunately increases the component count, and possibly also increases the size, of the meter. Another calibration technique which does not increase the size or complexity of the meter is laser trimming of film resistors. However, this type of calibration requires very expensive equipment for implementation and does not permit subsequent readjustment by the customer.